1. Field of the Invention
This invention relates generally to imaging in a photo-imageable polymer film on a semiconductor wafer, and more particularly to forming a structural feature, for example, a via, partially through a thickness of the polymer film.
2. Description of the Related Art
A wafer-level chip scale package (CSP) is a package for an integrated circuit that is substantially the size of the integrated circuit or of a flip chip, which uses a wafer-level processing technique. Unlike a flip chip, the wafer-level CSP has one or more passivation layers on the active side of the die. Each passivation layer typically comprises a layer of photo-imageable polymer film. The wafer-level CSP is smaller than a standard ball grid array (BGA), typically uses metal traces of a re-distribution layer (RDL) to route solder ball pads to standard pitches, and uses CSP-size solder balls on the re-routed pads. A wafer-level CSP uses a standard surface mount technology assembly process that is also used for BGAs, and does not require underfill.
When a polymer film is imaged, it is generally desired that a structural feature, such as a via or a hole, is achieved through the entire film thickness, or layer. All photo-imageable polymer film systems and processes, have some minimum size, or critical dimension, for example, ten (10) microns, that is resolvable to open the structural feature, or feature, completely through the layer. Assuming that the layer is in an x-y plane of an x-y-z coordinate system, a depth of the feature is in the z direction. A structural feature that penetrates completely through the layer of polymer film is a full-depth feature. To make a full-depth feature, the smaller of the dimensions of the feature in the x-y plane, must be larger than the critical dimension. A feature in which the smaller of the dimensions of the feature in the x-y plane is smaller than the critical dimension, is a partial-depth feature and does not penetrate completely through the layer.
The critical dimension is determined by many factors including the type of material comprising the layer, the thickness of the layer, the exposure tool used, several exposure process parameters such as exposure energy, exposure time and depth of focus, and several developing process parameters such as the developing solution, temperature and time.
A photomask, which comprises a UV light-blocking material, such as chrome, disposed on a transparent glass or quartz plate, is used to expose a polymer film. A prior art photomask has chrome disposed in patterns that have a shape and size of the features that are desired to be produced in the polymer film. In the case of a negative acting polymer film, polymer film exposed to light becomes cross-linked and cannot be developed, i.e., removed, from the exposed area. In the case of a positive acting film, the polymer film exposed to light is removed during development.
Using prior art methods, a partial-depth feature in which its smaller dimension is greater than the critical dimension cannot be formed in a polymer film at the same time, i.e., during a single series of photo-imaging steps using one photomask, as a full-depth feature is formed.